Development of novel therapeutics that can induce apoptosis by directly targeting the Bcl-2 family proteins has been a major focus of cancer research. Several groups have developed BH3 peptides derived from pro- apoptotic Bcl-2 family proteins or produced chemical compounds that could act as BH3 peptidomimetics, which could induce apoptosis by releasing pro-apoptotic Bax and/or Bak protein from inhibition by the anti-apoptotic proteins. Applications of BH3 peptides or their derivatives produced some successful outcomes in treating cancers. However, this approach seems to have limitations when it is applied to cancers displaying overexpression of anti-apoptotic Bcl-2 proteins or reduced Bax and Bak levels, because aberrant expression of these proteins cause resistance to such BH3 approaches. Alternative strategies that bypass regulation by Bcl- 2 family proteins must be considered. Previously we reported that ATAP, a novel amphipathic tail-anchoring peptide of Bfl-1, triggered potent mitochondria-dependent cell death. ATAP can directly form cytochrome c- releasing pores in mitochondrial membranes, and the pro-apoptotic activity of ATAP does not require and is not influenced by either anti- or pro-apoptotic proteins. We also found that ATAP is more effective in inducing apoptosis than the various BH3-peptides in many cancer cells. We used the internalizing RGD peptide (iRGD) for selective tumor targeting and delivery of ATAP into cancer cells. This ATAP-iRGD fusion peptide can be effectively synthesized, and similar to the native ATAP, ATAP-iRGD can target mitochondria permeability for induction of apoptosis. In contrast to ATAP which causes cell death indiscriminately, the ATAP-iRGD peptide appears to selectively target tumor cells for apoptosis. Moreover, ATAP-iRGD was effective at inducing cell death in the DU145 human prostate cancer cell line that was resistant to the effects of BH3 peptide therapy. Preliminary studies with intravenous injection of ATAP-iRGD into xenograft nude mice with human cancer cells could reduce tumors size while not producing significant off-target toxicity. These findings lead us to believe that ATAP can be an effective therapeutic agent for treatment of cancer cells that are potentially resistant to the existing chemotherapeutic reagents. This SBIR-Phase I application will leverage our expertise in biologic drug development to pursue necessary studies for development of ATAP-iRGD as a treatment for prostate cancer in an effort to move towards the filing of an Investigational New Drug (IND) application with FDA. This effort will contain the following two specific aims. Aim 1, test the pharmacokinetic, tissue distribution and toxicological profile of ATAP- iRGD in a mouse model. While our preliminary results suggest limited toxicity associated with ATAP-iRGD, further toxicology studies to evaluate the safety profile of ATAP-iRGD are essential to enable IND filing. Completion of these evaluations will provide an essential guidance for our in vivo animal model studies testing the efficacy of ATAP-iRGD in treatment of prostate cancer. Aim 2, establish the efficacy of ATAP-iRGD in a xenograft mouse model of prostate cancer. Here we will test if various doses of ATAP-iRGD can affect tumor growth in xenograft mouse model with various human prostate cancer cell lines. These efforts will include comparative studies to the efficacy of BH3 peptides in reducing tumor growth. We will collect data on tumor size as well as use histology to grade the progression of the tumor following treatment with ATAP-iRGD or BH3 peptides. We will also conduct additional distribution studies using the xenograft mouse models. Fulfillment of the above studies will provide proof-of-principle data for our future extensive effot toward developing ATAP-iRGD as a novel therapeutic reagent for treatment of cancer cells that are resistant to the existing chemotherapeutic reagents due to the aberrant expression of Bcl-2 family proteins.